The laser ultrasonic inspection apparatus includes an ultrasonic excitation unit with a laser and an ultrasonic detection unit with a laser. The ultrasonic excitation by the laser is performed by irradiating test pieces with a pulsed laser beam (excitation laser). When the power density of the laser beam is low, thermal stress is generated due to the rapid heating-cooling process on a micro region of the surface, and the generated thermal stress serves as the source of distortion of the material, whereby ultrasonic signals are generated (a thermoelastic mode).
On the other hand, when the power density of the laser beam is high, a surface layer of the test piece is turned into plasma, and the pressure is applied to the test piece as a reaction to the expansion of plasma, whereby ultrasound is generated (an ablation mode).
The reception of ultrasonic waves by a laser is performed by measurement of the surface displacement induced by ultrasonic waves with a laser displacement meter. As the laser displacement meter, there is a method of using deflection of beam by surface displacement which is called a knife edge method. The knife edge method is inexpensive, and a speckle knife edge detector (hereinafter, referred to as SKED) capable of being applied to a rough surface is disclosed in JP-T-2015-505362. Therefore, a practical and inexpensive laser ultrasonic inspection apparatus can be realized using a knife-edge type detector.
An example of related art includes JP-T-2015-505362.